An agricultural machine of the planter type is driven, usually towed by a tractor, across the soil that is to be sown, and embraces a plurality of drilling units distributed in the lateral direction of the machine, each one of which drilling units is arranged to form a drill furrow in the direction of travel of the machine and, by a special device, place a row of seeds in the drill furrow. In order to provide good opportunities for the crops and the harvest, it is important that the seeds are placed at a constant, uniform distance from each other. There is also a desire to be able to keep a high speed of motion upon sowing, since the time consumption decreases thereby.
U.S. Pat. No. 4,450,979 discloses a seed metering device for a planter having drilling units, each one of which has two rotatable discs inclined in opposite directions for the formation of a drill furrow, which drilling units can move vertically in relation to the frame of the planter. The seed metering devices, each one of which is fixedly connected with a respective drilling unit, are individually arranged to distribute one seed at a time to the respective drill furrow. The seed reception on the seed metering element is effected by a positive pressure on the seed side of the seed metering element. The positive pressure is separated by a brush seal as well as a baffle seal from the area where the seed releases from the seed metering element to fall down through a seed chute to the drill furrow only by means of the gravity.
In solutions of this type, disclosed in the document above, not infrequently a non-uniform distance distribution of seeds along the drill furrow is obtained, particularly upon relatively high speeds of the machine. The non-uniform distribution is caused by the fact that the seed is released at an initial speed from the seed metering element to fall by the gravity to the seed chute and through the same slide down to the drill furrow in the ground. When the seed releases from the seed metering element, there is a variation of the initial speed of the seed toward the ground depending on the vertical motions of the drilling unit and thereby of the seed metering device when driving over the irregularities of the field. The variation of initial speed results in a variation of the transportation time of the seed from the seed metering element to the drill furrow.
SE0700529 shows a solution where, by varying the speed of rotation of the seed metering element depending on the vertical motions of the seed metering device, the variation of initial speed is decreased.
Another factor of variation of the transportation time is that certain seeds bounce against the wall of the seed chute, on one hand depending on the vertical motions of the seed chute when driving over the irregularities of the field, and on the other hand depending on a variation of the fall angle of the seed, in the free fall, down into the seed chute. Variations of the transportation time from the seed metering device to the drill furrow results in a non-uniform distance distribution of seeds along the drill furrow.
Such problems described above with non-uniform distance distribution of seeds in the drill furrow may imply that the speed of motion upon sowing has to be kept relatively low